Which statement correctly describes the set of all antiderivatives of a differentiable function f on an interval?
a) If F′(x)=f(x), then the only antiderivative of f is F(x); the constant term is fixed by f.
b) Any two antiderivatives of f differ by a function whose derivative is 1 on the interval.
c) If F′(x)=f(x), then every antiderivative of f can be written as F(x)+C, where C is any real constant.
d) If F′(x)=f(x), then every antiderivative of f can be written as F(x)+C(x), where C(x) is any differentiable function.